renderer/opengl: simplifying the custom shader to get things working

src/renderer/opengl/CustomProgram.zig

@@ -7,6 +7,12 @@ const gl = @import("opengl");
 
 program: gl.Program,
 
+/// This VAO is used for all custom shaders. It contains a single quad
+/// by using an EBO. The vertex ID (gl_VertexID) can be used to determine the
+/// position of the vertex.
+vao: gl.VertexArray,
+ebo: gl.Buffer,
+
 pub const Uniforms = extern struct {
     resolution: [3]f32 align(16),
     time: f32 align(4),
@@ -33,24 +39,48 @@ pub fn createList(alloc: Allocator, srcs: []const [:0]const u8) ![]const CustomP
 }
 
 pub fn init(src: [:0]const u8) !CustomProgram {
+    _ = src;
     const program = try gl.Program.createVF(
         @embedFile("../shaders/custom.v.glsl"),
-        src,
+        //src,
+        @embedFile("../shaders/temp.f.glsl"),
     );
     errdefer program.destroy();
 
     // Create our uniform buffer that is shared across all custom shaders
     const ubo = try gl.Buffer.create();
     errdefer ubo.destroy();
+    {
         var ubobind = try ubo.bind(.uniform);
         defer ubobind.unbind();
         try ubobind.setDataNull(Uniforms, .static_draw);
+    }
+
+    // Setup our VAO for the custom shader.
+    const vao = try gl.VertexArray.create();
+    errdefer vao.destroy();
+    const vaobind = try vao.bind();
+    defer vaobind.unbind();
+
+    // Element buffer (EBO)
+    const ebo = try gl.Buffer.create();
+    errdefer ebo.destroy();
+    var ebobind = try ebo.bind(.element_array);
+    defer ebobind.unbind();
+    try ebobind.setData([6]u8{
+        0, 1, 3, // Top-left triangle
+        1, 2, 3, // Bottom-right triangle
+    }, .static_draw);
 
     return .{
         .program = program,
+        .vao = vao,
+        .ebo = ebo,
     };
 }
 
 pub fn deinit(self: CustomProgram) void {
+    self.ebo.destroy();
+    self.vao.destroy();
     self.program.destroy();
 }

src/renderer/shaders/custom.v.glsl

@@ -1,7 +1,8 @@
 #version 330 core
 
-layout (location = 0) in vec2 position;
-
 void main(){
-    gl_Position = vec4(position.x, position.y, 0.0f, 1.0f);
+    vec2 position;
+    position.x = (gl_VertexID == 0 || gl_VertexID == 1) ? 1. : 0.;
+    position.y = (gl_VertexID == 0 || gl_VertexID == 3) ? 0. : 1.;
+    gl_Position = vec4(position.xy, 0.0f, 1.0f);
 }

src/renderer/shaders/temp.f.glsl

@@ -0,0 +1,8 @@
+#version 330 core
+
+layout(location = 0) out vec4 out_FragColor;
+
+void main() {
+    // read
+    out_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+}